The Threat of Red Britain

Tory Prime Minister Boris Johnson has, to his credit, seized the initiative in the battle over whether Britain will truly exit the EU, and on what terms.

But no one can know how this high-stakes gamble will turn out. Johnson just lost his slender parliamentary majority, and the prospect of a new election looms. If things break the wrong way, the winner could be opposition leader Jeremy Corbyn, a throwback leftist redolent of the bad old days of Britain's self-imposed stagnation.

It's hard to exaggerate the threat represented by Corbyn and Co. taking control of our most important ally. In US...